# Thread: Question on how to calculate depth of cut for a Octagon barrel

1. Junior Member
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## Question on how to calculate depth of cut for a Octagon barrel

Can somone help me with how they go about calculating the depth of cut for a octagon barrel say the overall blank diameter is 1.250, and you want to end up with say .900 across the flats. how much do you take off each of the eight sides and how do you go about figuring that out?

Straight barrel no taper.

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(1.25-.9)/2

3. Junior Member
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Thanks for the info. When I was searching I kept running into sites there talking about a constant factor of .9239 and a cosign of 20 etc etc so my head was full and I sure didn't want to take it down to far.

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Originally Posted by Dan Dubeau
(1.25-.9)/2
What he said. LOL.

Now if you want to know if that will leave you with sharp edges or a slight radius edge then you can download an arcs and chords calculator on your cell phone. The basic math you do yourself, or do it in CAD.

5. elf
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You'll need a welder or a 3d printer to make .9" flats on a 1.25" diameter octagon:

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He said .900 across the flats, not .900 flat width. I think most of us understood that pretty clearly.

7. elf
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The answer is still wrong no matter which way you measure

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????

like the man said, (1.25-.9)/2 - 1.75 off each side of the barrel.

Of course, I am assuming he's starting with a round barrel; looks like you are assuming he's starting with a 1/25" across flats octagon barrel.

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Is the barrel blank round, or octagon to start with? You said diameter in your first post.....

Last edited by Dan Dubeau; 04-12-2019 at 08:36 PM.

10. Junior Member
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My blank is 1.250 diameter round.

got it set up in my mill, will start milling in the morning, added a little height on the tailstock for about .005 taper per inch. Length will be somewhere between 20-22 inches.
Last edited by Mike8623; 04-12-2019 at 08:42 PM.

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